The present invention generally relates to a paper coating formulation having a reduced amount of binder as compared to conventional paper coating formulations and methods for making such paper coating formulations. More particularly, the present invention relates to binder for use in paper coating formulations comprising a strongly associated adhesive material, which binds to other coating components, such as pigments, that is less prone to depletion from paper coating formulations into a paper web during use. Specifically, the strongly associated adhesive material prevents other components of the paper coating formulation from moving from the coating formulation into the paper web during the paper coating process. As a result, a reduced level of total binder is needed for the paper coating formulation.
Pigment containing paper coating formulations provide paper with a desirable finish, gloss, and smoothness. A pigment containing paper coating is typically an aqueous mixture of pigment(s) and a binder or adhesive, which is applied to paper. The pigment in the coating fills in irregularities in the paper surface, produces an even and uniformly absorbent surface for printing, and improves the overall appearance of the coated sheet.
The binder or adhesive influences the properties of the coating mixture and the properties of the final coated paper. For example, the binder functions: (1) to impart the required rheology behavior and water retention to the coating mixture; (2) to bind the pigment particles together in the dried coating and to the paper web; and (3) to control the absorption of printing ink during printing on the paper.
The unique functional properties of paper are derived through the paper making process and coating formulations. Predictable performance for the end user is critical, and depends on a consistent composition of the basesheet and coating layer. One major complaint of coated paper producers is that paper properties are inconsistent. Variations in sheet gloss, pick strength, and fiber coverage appear in some cases to be time dependent; with acceptable properties achieved early in a paper processing run and declining as the run continues. Reduction in gloss potential can result in decreased productivity through finishing processes. Poor fiber coverage and pick strength can bring complaints from printers.
One theory concerning declining properties is that the composition of the paper coating can change over time as a result of application and metering processes. Specifically, paper coatings begin to lose water content as soon as they are applied to a paper web. The extent of the loss is dependent upon the absorbency of the substrate, the pressure applied to the coating during application, and the length of time between application and metering of excess coating in the process. Additionally, coating components that are soluble in water or highly mobile are depleted from the coating over time, a result of moving with the water into the paper web.
Consequently, the composition of excess coating returned to the machine supply can differ significantly from that which was originally applied. The binder, in particular, can rapidly deplete and must be consistently replenished by the addition of fresh coating to the supply.
From a practical standpoint, depletion of the binder is economically undesirable. Furthermore, it is desirable to use the minimum amount of binder required to obtain satisfactory cohesion of coating components and their adhesion to paper. Any amount of binder used above the minimum amount is generally objectionable as it leads to reduced brightness and hiding power, more difficult calendering of the paper, and increased costs.
As such, a need exists in the industry for a paper coating that is strongly-associating with other coating components thus reducing binder depletion during the paper coating process. Additionally, it would be desirable if the paper coating formulation comprised a reduced level of total added binder to reduce costs.